This invention relates to web surface inspection and, more particularly, to a method and apparatus for detecting the presence and location of defects of a photosensitive layer on a film web.
In general, the production of photographic roll films includes processes of sequentially applying multiple layers of photosensitive material onto a wide web of film base, winding the film up in a roll, and then slitting and cutting it into a plurality of film strips with a desired width and length while moving it lengthwise. When the application of photosensitive material is effected while moving a wide web of film base lengthwise, it is difficult to apply a uniform layer of photosensitive material to lateral margins of the wide web. For this reason, a certain width of the lateral margins will be slit away from the wide web in said slitting process and discarded.
However, the remaining major part of the wide web from which a plurality of strip films are made sometimes also has defects in the layer of photosensitive material. These defects, in the majority of cases, occur also in the lateral margins. Therefore, it is necessary to inspect the lateral margins in order to detect such defects in the remaining major part of the wide web. For this purpose, heretofore, the lateral margins slit away from the web in the slitting process have been retained and wound up for each web roll for subsequent visual inspection in order to find defects in the layer of photosensitive material which can be regarded to have spread to the major part of the wide web and, if found, to identify their locations. By such the visual inspection, however, it is difficult to identify the corresponding exact locations on the remaining major part of the film strip which has been wound up. As a result, it is hardly avoidable to cut away excessive areas of the web or film strip including defects. Because visual inspection must be effected for each web roll, the processes following slitting will be delayed. Furthermore, there is a variation in the results of visual inspections, between different inspectors.
For the purpose of automatically effecting such web surface inspections, there is known in the art a technique by which the defects of the application of photosensitive material layer to the web of film base are found by illuminating the web surface to be inspected. Such a technique is disclosed in, for example, Japanese Utility Model Publications Nos. 45-23978 and 46-25437, and Japanese Patent Unexamined Publications Nos. 58-68651 and 58-68652. This technique is characterized by inspecting the entire width of a wide web, resulting in a reduced inspection accuracy. Particularly, when this technique is applied to the inspection of photosensitive film webs, not only is the selection of an illumination source restricted owing to the photosensitivity of the film web, but also it is difficult to produce an output signal with an increased signal-noise (SN) ratio owing to the use of a low intensity of illumination light. Furthermore, there is a great possibility that defects occurring only in the lateral margins to be slit away may be incorrectly identified as occurring in the remaining major part of film web. Such misrecognition is avoidable if the web is inspected after the lateral margins have been slit away. In this case, two processes are required; one for slitting away the lateral margins from the web, and the other for slitting and cutting the remaining major part to strip films with a desired width and length.